Typically, there are two types of chucks that are used in semiconductor processing of substrates such as a wafer or a mask. The first type of chuck is known as a vacuum chuck, which employs a vacuum to hold the substrate in place against the chuck. The second type of chuck is known as an electrostatic chuck, which applies an electric potential between the substrate and the chuck to secure the substrate in place on the chuck during patterning of the substrate, or, in the case where the substrate is a mask, during patterning of a chip or wafer or the like with the chucked mask. Electrostatic chucks utilize the attractive force between the two plates of a capacitor to hold the wafer in place. If the wafer is separated from a parallel electrode by an insulator of dielectric constant .epsilon. and thickness d, and a voltage V applied between them, an attractive force F is generated between them as follows: EQU F=(.epsilon.V.sup.2 /2d.sup.2)A
where A is the common area of the wafer and electrode. Clearly the maximum force is achieved by using a thin dielectric layer with a high dielectric constant. The electric potential is maintained during processing such that the chucked substrate or mask is held in place with a precisely maintained and controlled position. After patterning, the chucked substrate or mask can simply be removed by disconnecting the electric potential. A third type of chuck uses mechanical clamps to hold the wafer in place. However, the risks of damage to the patterned wafer and particle generation increasingly discourage the use of mechanical clamps.
The use of electrostatic chucks is the preferred method in vacuum systems, where vacuum chucks are obviously not applicable. For high resolution lithography applications a serious problem is caused by particles trapped between the wafer and the top surface of the chuck, because the particles can distort the top surface of the wafer, leading to distortions in the developed image in the resist coating on the wafer. In vacuum chucks, this problem is minimized by forming the top surface of the chuck like a "bed of nails," leading to a very low fraction of the surface of the chuck contacting the wafer. The result of this method is that the probability of a particle lying directly between the wafer and a contacting part of the chuck is small.
The "bed of nails" solution for an electrostatic chuck is difficult. The separation between the wafer and electrode d must exceed the largest particle anticipated, putting a lower limit on d. Also, assuming the dielectric makes up the "nails," the effective dielectric constant is reduced. If the nails represent a fraction f of the area of the wafer, then the effective dielectric constant is f.epsilon.+(1-f), since the dielectric constant of free space is 1. If, for example, .epsilon.=3 and f=0.05, the effective dielectric constant for the "bed of nails" is 1.1 rather than 3. Thus, the hold down force is significantly lower for the "bed of nails."
Another problem associated with vacuum operation is that it is difficult to conduct heat away from the substrate as it is heated by the exposing radiation. The normal construction of wafer chucks is not conducive to efficient heat transfer. Also, for some electron-beam lithography applications, electron optics considerations require the substrate and its local environment to be immersed in a magnetic field. If the substrate chuck is mounted on a stage which moves during exposure, and if the chuck or stage is constructed of materials with high electrical conductivity, aside from very thin conducting films, eddy currents can be generated which will perturb the magnetic fields and possibly disturb the exposure process. This requires that the chuck and stage be manufactured from non-conducting materials, such as ceramic materials. However, these materials typically have low thermal conductivity.
In some applications where precise locations of features on the wafer are important, such as lithography applications, the wafer may absorb enough heat to cause local thermal displacements, or distortions, of the features on the wafer. These displacements may be enough to create overlay errors between different layers of patterns on the wafer. In conventional vacuum chucks, the hold-down force has been sufficiently large so far to prevent these displacements. This may not be true for electrostatic chucks. The wafer then may expand, and as it expands it may break loose from the chuck locally and slip on the surface of the chuck. This "stiction" can cause unpredictable location errors. In principle, the thermally induced wafer distortions can be predicted, and corrections can be applied to the lithography imaging system. However, if stiction is present, the timing and amount of the displacements can not be accurately predicted.
Accordingly, there is a need for electrostatic chucks that are vacuum compatible, tolerant of particles, have high thermal conductivity, and avoid problems caused by stiction.